Hot water is easy to forget during design, yet a cottage uses it every day for showers, dishes and laundry. In a small home, the water heater also competes for scarce closet space. DADU water heaters come in several types, and Seattle’s energy and residential codes shape where they go and how they score.

This guide covers the main equipment types, location rules, energy credits and permits. It sticks to what the codes say, so you can compare options with your designer and plumber.

Why DADU Water Heaters Deserve Early Attention

After all, a water heater affects more than comfort. It ties into the electrical load, the energy credit plan, the plumbing layout and the floor plan. In a compact cottage, a tank in the wrong place can take over a closet or crowd a laundry nook.

So it pays to choose the type before the layout is final. Then the designer can reserve the right space, and the electrician can plan the circuit. For storage ideas in tight plans, see our guide to DADU storage planning.

Common Types of DADU Water Heaters

Most cottages use one of a few common types:

• Heat pump water heaters that move heat from the surrounding air into a storage tank
• Split heat pump systems with the heat exchanger outdoors and the tank inside
• Electric resistance tanks that heat water with elements
• Tankless units that heat water on demand
• Gas fired heaters for lots that will have gas service

Of course, each type has trade offs in space, power, venting and code credits. Therefore, the best choice depends on the full design, not on one feature alone.

How the Energy Code Scores Hot Water

Seattle enforces the 2021 Seattle Energy Code. Under Section R406, each new dwelling unit must earn a set number of energy credits, and water heating is one of the categories. The code lists gas options, electric heat pump options and a split heat pump option. In that list, the electric heat pump choices earn more credit than the gas choices.

Two other options can stack with any water heater. One is a drain water heat recovery unit that captures heat from shower drains. The other is a compact hot water distribution design that limits how much water sits in the pipes.

To claim a credit, the permit drawings must name the option. For the water heater options, the drawings must also list the equipment type and its minimum efficiency. You can review the rules on the SDCI energy code page.

Where the Water Heater Can Go

Next, location rules come from both codes. The energy code says service hot water systems shall go within the building thermal envelope. However, it lists exceptions, including tankless water heaters, gas heat pump water heaters meant for outdoor use, atmospheric vented gas heaters and systems that meet a high efficiency rating.

The residential code adds safety limits. According to Section M2005, a fuel fired water heater cannot sit in a room used as a storage closet. Also, a water heater in a bedroom or bathroom needs a sealed enclosure so it does not draw combustion air from the living space, though direct vent units do not need the enclosure.

Heat Pump Rules and Backup Heat

Heat pump water heaters often include a backup electric element. The energy code limits when that element may run. For example, it may run when the heat pump cannot meet demand, during defrost or if the heat pump fails. For units in unconditioned space, it may also run when outdoor air is cold, as the code defines.

In addition, the code addresses tanks on cold floors. A tank type heater in unconditioned space, or on a concrete floor inside, must sit on an insulated surface. These details can affect the size of the closet and the base under the tank.

Power and Plumbing Coordination

Electric water heaters add load to the panel. As a result, they belong in the load calculation along with heat, cooking and laundry. Our post on the DADU electrical upgrade explains how that math guides panel and service choices.

Likewise, the plumbing layout matters. A water heater near the bath and kitchen keeps pipe runs short, which also helps with the compact distribution credit. For the supply side, our guide to DADU water service covers line sizing and the shared meter.

Permits for DADU Water Heaters

In Seattle, Public Health Seattle and King County issues plumbing and gas piping permits. Its permit page says that whenever you install, relocate or change a plumbing or gas piping system, Seattle and King County codes require a permit. In practice, that covers a new water heater and its piping. A homeowner may also apply for a permit to do their own plumbing, though Public Health recommends hiring a professional plumber.

Meanwhile, SDCI’s mechanical permit covers heating, venting and air conditioning, but not plumbing or gas lines. So a cottage project may involve several permits for different systems. The Public Health plumbing and gas permit page explains how to apply.

Choosing the Right Setup

First, think about how people will use the cottage. For example, a guest suite and a full time rental may need different capacity. Next, list the fixtures and appliances that need hot water. Then ask the designer to compare each type against space, credits, power and venting.

Codes change over time, so confirm the current edition before you buy equipment. Overall, DADU water heaters work best when the type, location and permits come together during design, not after the walls close.